PM says chemical weapons attack on civilians 'cries out for a strong response'.

Prime Minister Malcolm Turnbull has strongly backed U.S. military action in Syria in response to this week's "abhorrent" chemical weapons attack, but made it clear that Australia would not yet be throwing its weight behind any campaign to combat the Assad regime.

The U.S. launched 50 missiles at an airbase near the city of Homs on Friday morning (AEST), the site where the chemical attack is though to have originated. Shortly after the missile strike was reported, Donald Trump gave an address calling on other nations to join a U.S.-led campaign in Syria.

Earlier in the morning, hours before the attack, Turnbull and defence minister Marise Payne signalled Australia would consider action in Syria, if asked by the United States to assist. Following the missile strikes, Turnbull said the U.S. had not requested Australian military support, and that Australia was informed of the strike prior to it being carried out.

"We are not at war with the Assad regime and United States have made it clear that they are not seeking to overthrow the Assad regime," the PM said at a press conference in Sydney on Friday afternoon.

"The United States advised us as a Coalition partner shortly prior to the attack... the United States secretary of defence, Jim Mattis, phoned [Payne] earlier this morning.

"This was a calibrated, proportionate and targeted response. It sends a strong message to the Assad regime. The use of these weapons, under any circumstance, is illegal and abhorrent. It is a violation of international law. It is a war crime."

Opposition leader Bill Shorten said in a statement that Labor backed the strike.

"Labor supports the U.S. military strike against the Shayrat Airfield and military depots as appropriate and proportionate, targeting the Assad Regime's capability to conduct gas attacks against its civilian population. We support the US sending a strong signal that these gas attacks should have never occurred - and they should never occur again," Shorten said.

"It was an atrocity, a war crime against civilians. The illegal and abhorrent use of chemical weapons by the Assad regime against its own people demanded a strong response."

"I don't want to speculate any further about that. You know where we stand," he said.

"We have condemned this attack, utterly. It cries out for a strong response and we are in very close touch, as we always are, constant communications with our allies, in particular the United States."

On ABC radio on Friday morning, Payne too condemned the attack, and while she stopped short of committing to any action, the Defence Minister said Australia would be "part of the conversation" on any international military response to the chemical attack.

"We have to be clear with Russia, we have to be clear with those who support the regime that this is totally unacceptable," Payne said.

"The U.N. security council, I understand, is considering the options that are available to it and we would call on them to take action in response to that."

Marise Payne says we need to be very careful in dealing with the Syria situation. Australia will be part of any conversation on responses

When asked if Australia would join U.S. action without backing from the United Nations, Payne said "we'll make decisions in regard to those sorts of issues as they are brought to us".

"I'm not going to comment on the hypothetical."

On 3AW radio later in the morning, Prime Minister Malcolm Turnbull also criticised the attack and President Assad. He called it a "war crime of the worst kind" but again stopped short of committing Australia to any action.

Turnbull says Russia hasn't yet acted appropriately in response to the Syrian attack, he describes it as heartbreaking #auspol